Ordering items in subcategories - alternating subcategories

I get a "500 - An error has occurred!" when trying to organize the order of sub category items within another category.

So for example, my category structure is like this:

cat1
|_ cat2
| |_item1
| |_item2
|_cat3
| |_item3
| |_item4


(note: 0 items in cat1)

but when I click on view items by cat1 and apply filters on include subcategories items I see the categories listed as:

item1 - cat2
item3 - cat3
item2 - cat2
item4 - cat3

I would have expected that the default behaviour to be to order the items based on the order of the subcategory and then each items as it is ordered within that subcategory

When trying to change the order and then save in this view, i get the error 500 message.

Is there a way to make sure that items from cat2 appear one after another and before items in cat3?

Thanks for your help!